Research Brief #26

Interaction of Shear and Tension-Perpendicular to Grain in Southern Pine Dimension Lumber
Louisiana Forest Products Development Center
Louisiana State University
Baton Rouge, LA

Research Objectives

• Investigate the interaction of shear and tension perpendicular-to grain stresses in Southern Pine dimension lumber.
• Quantify the relationship between horizontal shear (and rolling shear) and tension-perpendicular-to-grain strengths by developing empirical equations based on the experimental data.
• Investigate the validity of Norris' interaction equation for combined shear and tension perpendicular to grain in Southern Pine and recommend future research in this area.

Synopsis of Results

• The average pure shear strength, rolling shear strength and pure tension-perpendicular-to-grain strength were around 1600 psi, 520 psi and 460 psi, respectively.
• The average values of shear strength (and rolling shear strength decreased with increase in the tension-perpendicular-to-grain stresses.
• Norris' interaction equation was found to overestimate the strength of the material for all ranges of shear and tension perpendicular to grain stresses.
• Regression equations which expressed the shear strength as a function of tension-perpendicular to grain stress were obtained based on the experimental data. Similarly equations were developed to express the tension-perpendicular-to- grain strengths as a function of the shear stress.

Conclusions

The average shear strength decreased with the increase of tension perpendicular-to-grain stress and vice-versa. The results of the study clearly demonstrated the need to account for the interaction of these stresses in design. Norris' interaction equation was found to overestimate the strength of the wood material subject to the combined stress condition. The need for a careful examination of the interaction behavior in other species and glued laminated timber was established.

Researchers

Runwei Zheng, Research Assistant
Department of Civil and Environmental Engineering.

Vijaya K.A. Gopu, Professor and Coordinator
Structural Engineering & Mechanics Program,
Department of Civil and Environmental Engineering, LSU

Target Group

Residential and commercial housing builders
Louisiana economic development and public policy officials.

Methodology

• The tension-perpendicular-to-grain stress on shear strength of Southern Pine dimension lumber was evaluated experimentally. The test program was designed to replicate a given combination of shear and tension-perpendicular-to-stresses three times.
• Regression analysis of the experimental data was conducted to develop interaction equations which predict the behavior of the dimension lumber under a combined stress condition.
• Validity of Norris' interaction equation was tested by comparing the predicted strength values with the experimentally obtained data and the values predicted by the regression equations.